A transceiver both transmits and receives signals. Both the transmitter and the receiver can have signal processing impairments. Impairments can include path imbalance and oscillator leakage. Both types of impairment can degrade the transmitter's generation of an output signal or the receiver's processing of an input signal.
Path imbalance is an impairment due to unequal performance of an inphase path (I-path) or a quadrature path (Q-path) of a transmitter or receiver. Path imbalances can therefore be called inphase-versus-quadrature (I-Q) imbalance. I-Q imbalance can be caused by either a gain difference or a phase difference between the two paths.
Oscillator leakage is an impairment that can occur when a portion of an oscillator signal feeds through from an input to the output of a mixer. A mixer multiplies two signals, such as an oscillator signal and a signal to be either upconverted or downconverted to a higher or lower frequency, respectively. Neither the oscillator nor the convertible signal should appear in the mixer output, only the product.
Oscillators that are used with mixers are called local oscillators (LO). Since both the transmitter and the receiver use imperfect mixers, a portion of the oscillator signal can appear in a mixer output; i.e. these mixers exhibit LO leakage.